In recent years, there have been increasing demands for thin-type speakers, for the reason that they should be mounted in thin-type displays, and the like. Therefore, various types of thin-type speakers have been contrived.
Patent Document 1 describes a speaker structured to have a flat-plate type PolyVinylidene DiFluoride (PVDF), and electrodes formed on the opposite main surfaces thereof.
However, such conventional thin-type speakers have had the drawback of poor sound-quality characteristics, in general, since they cannot have larger depthwise sizes as in dynamic speakers.
In order to overcome this drawback, for example, a speaker in Patent Document 2 is adapted to oscillate a flat-plate type membrane made of a resin and the like, through an electromagnetic type exciter (actuator). The speaker in Patent Document 1 has an electromagnetic-type exciter which is mounted to a side surface of a membrane.
Further, a speaker in Patent Document 3 is adapted to have two flat plates placed with a predetermined interval interposed therebetween, and electromagnetic-type exciters (actuators) placed in a hollow area between these flat plates, such that the flat plates are oscillated by these exciters.    Patent Document 1: JP-A No. 2009-272978    Patent Document 2: JP-A No. 62-73898    Patent Document 3: JP-A No. 2005-117217